Big cams (lots of overlap) create idle problems having to do with "reversion" issues.
They allow (and actually cause) exhaust gas to enter the open intake valve and seriously dilute and adulterate (if, you will,) the gas/air mixture to the point that is becomes almost non-combustuble.
Advancing the spark is a crutch for this condition because slow-running engines LIKE a lot of advance, and it helps generate torque, which makes the idle seem a little less iffy. but the problem is still there. The only fix I know of is to run wider lobe separation to minimize over-lap, but that comes at a price (power-production.) The only for sure cure I know of is to run some sort of forced induction which will minimize the necessity for long-duration cams profiles. One more GOOD reason to turbocharge a slant six.
